Standard emollients for the cosmetic, toiletry and personal care industries have been esters (monohydric) such as isopropyl myristate, butyl stearate, cetyl octanoate and isostearyl isostearate covering a wide breadth of molecular weights. Monohydric esters have also been manufactured from high molecular weight compounds such as behenic acid and behenyl alcohol which yield a solid ester useful as an emollient for increasing the melting point characteristics of a given formulation. Synthetic spermacetic waxes have been utilized for the past thirty years to replace the natural rare variety. Such products as cetyl palmitate, cetyl myristate and mixed cetyl esters have essentially replaced the use of spermacetic wax.
In the mid-seventies, there was a critical shortage of beeswax, and the synthesis of a synthetic version of this natural wax and emollients derived from this wax was sought. As one solution, the relatively high molecular weight dodecanedicarboxylic acid was esterified with a mixture of cetyl, stearyl and polyoxyethylene glycols to render a product that was found to be a suitable replacement. With the mixture of relatively high molecular weight alcohols and polyethers, the dicarboxylic acid of C.sub.12 yielded a perfectly usable product for skin applications-where natural beeswax had been used for numerous years.
For many years, monohydric alcohol diesters used in the cosmetic and personal care industries have been synthesized from such difunctional carboxylic acids as adipic, sebacic, azeleic, dodecanedicarboxylic acid and dimer acid, as well as anhydrides such as maleic, succinic and phthallic, among others. Based on the molecular weight of the difunctional carboxylic acid and the alcohol used, it was possible for the synthetic organic chemist to obtain a wide range of properties from these compositions, including, for example, dryness, oiliness, spreadability, lubricity, freezing point depression, insect repellency (e.g., dicapryl adipate), melting and slip point modification, emulsion synergism, solvency and color dispersion.
As the molecular weight of the dicarboxylic acid and corresponding alcohol increase, a greater degree of heaviness is imparted to the emollient, along with a higher degree of viscosity. As the degree of unsaturation of the alcohol used increases and becomes more olefinic in character, spreadability is enhanced. For example, diesters derived from oleyl or linoleyl alcohols have a greater degree of spreadability than does a corresponding diester derived from stearyl or isostearyl alcohol.
It is possible to further influence the melting point and indeed the viscosity of a diester through the use of branched rather than linear structures. With the introduction of an ether linkage, the liquidity of a given diester is also enhanced. Diesters made from ethoxylated and/or propoxylated adducts of alcohols have also been used as emollients and emulsifiers.
As a result of molecular weight and structure, diisopropyl adipate has been found to be an extremely dry ester with excellent solvency characteristics. Because of its physical properties, diisopropyl adipate finds use in products ranging from floating bath oils, after shaves, creams, lotions, deodorants, pre-electric shave lotions, to antiperspirants. Diisopropyl adipate has been widely used for numerous years in these applications because of the availability of raw materials and relatively low production cost. Diisostearyl adipate, on the other hand, is a relatively heavy, water white diester with little or no odor that imparts a luster and sheen to the skin and, as a result of its molecular weight and viscosity, has a tendency to linger on the skin. Dicapryl adipate is a diester derived from natural sources (through the cracking of castor oil to yield caprylene, then conversion to the corresponding alcohol) which has been found to exhibit insect repellency and has met with huge success in repellents for use in human and animal products. Diesters derived from dimer acids, such as diisopropyl dimerate and diisostearyl dimerate, in addition to being excellent, long lasting emollients because of their relatively high molecular weight, were shown to add anti-irritation characteristics to given skin formulations.
Typical end-use applications of diesters of dicarboxylic acids in the personal care, cosmetic as well as toiletry industries include: skin care products, eye makeups, body shampoo, stick deodorants, protective skin formulations, lipsticks, lip glosses, pre-electric and after shave lotions, after-bath splashes, shampoos and rinses, presun and sun products, antiperspirants and sunscreens.